1. Technical Field
The present invention relates to a path allocation technique for use in a multipath system.
2. Background Art
Various attempts have been made to improve the efficiency of computer systems that include a host computer and a system resource (e.g., storage device) used by the host computer.
For example, Japanese Unexamined Patent Application Publication No. 2008-257572 discloses a method for creating multiple logical partitions by logically partitioning storage devices, controlling the logical partitions so that each logical partition separately operates as an independent storage system, and changing the proportion allocated to each logical partition according to the amount of access from a host computer. With this method, the resource of a storage device, which is owned by a logical partition having a small amount of access, can be allocated to a logical partition having a large amount of access. As a result, the storage resource can be effectively used.
There has also been known a multipath system in which a host computer and a system resource are connected through multiple paths. According to the multipath system, path redundancy or distribution of an input/output (I/O) load is realized. Various methods for the construction and operation of such a multipath system have been proposed.
For example, Published Japanese Translation of PCT International Publication for Patent Application, No. 2002-533831 discloses a method for a multipath system employing multiple paths simultaneously. The method has been devised for allocating I/O operations for transmission/reception between a host computer and a system resource to any of the multiple paths. Specifically, according to this method, a time to complete the I/O operation currently being performed by each path is predicted, and a next I/O operation is allocated to the path which is expected to complete the I/O operation most quickly. This makes it possible to improve the usability of multiple paths and increase the efficiency of the entire system compared with the conventional round robin technique in which I/O operations are sequentially allocated to multiple paths.
Additionally, Japanese Unexamined Patent Application Publication No. 2007-157089 discloses a method for a multipath system including a path (regular path) for ordinary use and a path (sub path) used when a failure occurs in the regular path. According to the method, when a failure is detected in the regular path, the regular path is switched to the sub path and the regular path is blocked until the failure in the regular path is resolved.
If the regular path is not blocked during the time when a failure occurs in the regular path, the access from a host computer to a storage device is executed by the following procedure. That is, “the host computer accesses via the regular path”, “the host computer cannot access the storage device because a failure occurs in the regular path”, and “the regular path is switched to the sub path”. According to the method disclosed in Japanese Unexamined Patent Application Publication No. 2007-157089, the failed regular path is blocked until the regular path is restored, and thus the host computer constantly “accesses the system resource via the sub path” during this period. This can improve the access efficiency during the period when a failure occurs in the regular path.
Moreover, there has also been known a multipath system in which a host computer having multiple logical partitions, which are configured for each of multiple operating systems (OSs), and a system resource are connected through multiple paths, and in which the multiple logical partitions share the system resource.
In the multipath system in which the host computer having multiple logical partitions and the system resource are connected through the multiple paths and in which the multiple logical partitions share the system resource, there is a fear that, when the multiple logical partitions have various amounts of I/O, processing in response to requests from logical partitions having a large amount of I/O accounts for a large portion of the processing of the entire system, and application processing in logical partitions having a small amount of I/O may be hindered.